SOCIAL ORGANIZAnON IN A POPULAnON OF THE HOODED CROW

JON LOMAN Dept. of Ecology, Ecology building, S 223 62 Lund, Received 16 February 1982, revised 8 May 1984

CONTENTS cally related, . Corvid social organization 1. Introduction...... 61 can roughly be classified as territorial, colonial, 2. Study area...... 61 or communal. Territorial systems are those 3. Methods...... 62 3.1. Trapping and tagging...... 62 where the living space is split up into exclusive 3.2. Recording...... 62 territories, at least during the breeding season. 3.3. Radio-tracking...... 62 Colonial species have their nests concentrated 4. Results...... 62 and feed in a common area around the nesting 4.1. Territorial and flock crows in spring...... 62 4.2. Acquisition of territories and pair bond...... 64 colony but each nest is tended by a single pair. 4.3. Attachment to the territory...... 65 Communal societies are those where a family 4.4. Seasonal variation in flock size and feeding group tends one nest in a shared territory. The stations 66 4.5. Distribution of crows during winter.. 68 Hooded Crow mainly belongs to the territorial 4.6. Migration...... 70 category. Abshagen (1963) has described how it 4.7. Survival...... 70 can adopt a colonial nesting strategy and here I 5. Discussion...... 71 will demonstrate that Hooded Crows show some 5.1. Segregation of territories and flock area...... 71 5.2. Pair formation...... 71 traits of communal nesting too. 5.3. Territory establishment...... 71 Some further data are included, but not di­ 5.4. Occupancy of territories during winter...... 72 rectly used for comparative purposes. This is to 5.5. Juveniles staying in their parents' territories...... 72 5.6. Distribution of feeding sites during winter 73 present a picture as complete as possible of the 5.7. Function of roosting behaviour 73 crow population studied. I believe this facili­ 6. Acknowledgements...... 74 tates understanding of phenomena to be dis­ 7. Summary...... 74 8. References...... 74 cussed. 9. Samenvatting..... 74 I have previously reported on the breeding bi­ ology of the crow population studied (Loman 1. INTRODUCTION 1977, 1980) and some data from these studies Data presented in this paper will be used for are given as a background. Eggs are laid in early two kinds of comparison that give information April and mean clutch size is 4.3 eggs. About on the evolution of social traits in the Hooded 10% of the clutches are lost or deserted. Some Crow cornix and other species. By com­ of these are replaced. Eggs hatch after an incu­ paring information from studies made at differ­ bation period of about 18 days. Hatching is ent localities, it is sometimes possible to eval­ asynchronous, spanning over 2-3 days. Twen­ uate the importance of environmental variables ty-five per cent of the broods are lost to preda­ for certain social traits. Other studies on aspects tors. A mean of 2.8 young fledge from the re­ discussed in this paper are, e.g. those by Tompa maining broods. The youngest one in a brood (1975) in Switzerland, Kalchreuter (1971a) and often starves to death. The number ofbreeding Wittenberg (1968) in West· and pairs in the study area varied between 39 and 52 Charles (1972) in . These studies refer (1.9-2.5 pairs per km2) during 1972-1979. to the C. carone. Although I pre­ fer to consider it a species different from the 2. STUDY AREA Hooded Crow, its ecological niche is sufficiently The study was conducted in southern Sweden (55 0 40' N, 0 2 similar to justify comparisons. 13 30' E). An "intensive study area" covered about 20 km of the Revinge area, a military training field. It was used for It is also possible to compare the social orga­ military training about 5 weeks per year and for the rest of nisation of crows with that of other, systemati- the year much of it was grazed by cattle. There were some Ardea 73 (1985): 61-75 62 SOCIAL ORGANIZATION HOODED CROW [Ardea 73 marsh-areas and a eutrophic lake, Krankesjon. All trapping m, using a 25x telescope. All crows received official, num­ and tagging was done in the intensive study area, where I bered leg rings in addition to the wing tags. This permitted attempted to find all nests. identification of crows found dead or killed by people out­ As all large winter roosts and some other places of impor­ side the study area. tance for tagged crows were situated outside the intensive study area, I also made observations in adjacent areas. Ag­ 3.2. RECORDING riculturalland dominated outside the Revinge area. Some Most observations of crows were made from a car. The places in the study area provided a large supply of food for data in the results section are based on all observations of crows throughout the year or during certain seasons. These tagged crows but the majority was made during two stan­ places will henceforth be referred to as "concentrated feed­ dard car routes in order to reduce bias. One, completely in­ ing places", CFP. CFP in the study area were: 1. A small side the intensive study area, comprised 40 km and was fenced area where pigs were kept in the open all year. The completed once every week from 1 April 1975 to 30 June crows had opportunities to take pig's food. 2. Two centres 1978. The second route ran up to 5 km from the intensive for cattle raising in the Revinge area. Cattle were always in study area, encircling it. This route comprised 70 km and the vicinity of these centres during winter and were fed with was completed once each week from 1 September 1976 to 30 hay and supplementary food in the open. The crows were June 1978. Although the standard routes were no longer often seen to feed among the hay. All calves were born in completed after this period, additional observations were the open at these centres in spring. Dead calves and after­ made during the period 1 April to 30 June 1979 in order to births provided important food for the crows. 3. Municipal determine the breeding status of tagged individuals. Each garbage tips. 4. A dung heap that was particularly favoured time a tagged crow was observed I recorded, among other by crows during winter. 5. Winter potato stores. These pro­ things the size of the flock in which the tagged was vided food for various months during winter. seen. Aggregation of crows were considered as flocks, if the The breeding crows were protected in the intensive study distance between individual was less than 50 m. area. I was always able to detect whether or not nests be­ longed to tagged crows in this area. Nesting by tagged indi­ viduals outside the intensive study area could sometimes be 3.3. RADIO-TRACKING recorded. Other common corvids in the study area were Some observations were obtained from radio-tracking C. frugilegus, Jackdaws C. monedula, seven crows. The transmitters used operated on the 27 MHz pica, and glandularius. The former two fre­ band. They had a range of 400-800 m, a life of about 10 quented the same winter roosts as the Hooded Crows. The days and weighed, about 30 g. Each crow was tracked for up density of Rooks increased considerably in the course of the to three full days. study but this did not seem to influence the density or distri­ bution of the Hooded Crow population. A pair of Hooded Crows that nested in a copse retained their nestsite despite 4. RESULTS the fact that Rooks started nesting in the same copse. 4.1. TERRITORIAL AND FLOCK CROWS IN SPRING

3. METHODS 4.1.1. The territorial system and territoryfidelity 3.1. TRAPPING AND TAGGING Most results of this work are based on spot observations Almost all crows could easily be classified as of tagged crows. Some supplementary information was either territorials or flock crows during the gained from continuous observation of radio-tagged individ­ months April to June. A crow that was always uals. Most crows trapped were taken in small, two-compart­ observed singly or in a pair within a restricted ment traps in their territories during the period March to area during this period,was considered a terri­ June. Trapping in the flock area was also done mainly dur­ torial crow; otherwise it was considered a flock ing this season. Altogether 60 territorial and.25 flock crows crow. These "labels" remained until the next were trapped during the springs of 1974-1979. A Norwe­ gian crow trap (Kalchreuter 1971b) was tried during some breeding season. winters. This was only successful for one short period in De­ During spring months, territorial pairs occu­ cember 1975 when 19 crows, most of them winter migrants, pied home-ranges that did not overlap but were were trapped. Most crows were tagged as nestlings,at the age of about 25 days. Altogether 230 nestlings were tagged probably more or less adjacent. The home­ during the springs .of 1975, 1976, and 1977. Of these, 136 ranges were therefore considered territories were still in the study area in July or later in the year of during the period April to June. Most of the ter­ hatching and 45 were present in July of next year. All crows trapped received wing tags (Picozzi 1971). The ritory was covered every day in search of food size of the tags was 33 x 70 mm. To facilitate identification and territorials usually roosted within their ter­ and to allow calculation of the rate of tag loss, identical tags ritories. Territorials were occasionally observed were used on both wings. For crows tagged as adults, the outside their territories during the breeding sea­ proportion of individuals losing both tags was calculated to be 0% after one year, 3% after two and 30% after three son: 1. Neighbouring pairs could sometimes be years. The tags could be identified at a distance of up to 300 seen feeding peacefully at the border between 1985] SOCIAL ORGANIZATION HOODED CROW 63 their territories. 2. Sometimes crows were seen making short excursions outside their territo­ ® ries. 3. If a territorial crow was trapped, neigh­ bours could be seen in its territory within a short period of time. A territorial crow was once 1km slightly wounded in a trap. After release it was attacked and probably killed by a neighbouring * Roost pair. 4. Territorial crows usually roosted within 8h Hour in area 50 Flock size their territory. However, four times tagged or radiotagged territorials were observed to parti­ cipate in evening gatherings either on the ® ground or in trees in their own of in neighbour­ ing territories. These gatherings probably in­ volved mainly territorials. After these assem­ blies, which lasted 15 to 30 min, the crows flew off single or in pairs. 4.1.2. Distributionand home-ranges of flock crows The home-ranges of flock crows were larger Fig. 2. Daily activity pattern of two radio-tracked flock than those of territorials during the period April crows. A was tracked on 18, 20 and 22 March 1977; on 18 to June and they overlapped widely. Flock March (broken line) during the afternoon only. B was track~ crows were mainly restricted to CFP and their ed on 5 April 1978. Duration of stay is not given when entire surroundings. Areas much used by flock crows day spent in one place. apart from CFP were probably situated at the intersection of several territories, far from the close to CFP and, at least in one case, the home­ nests (Fig. 1). These areas were utilized by the range of a nesting pair was regularly used by same flock crows as those usually present at the flock crows too. This pair was sometimes seen closest CFP. Nests were in some cases found feeding together with flock crows but as the pair

• Tagged flock crow O Nest o o A territorial female, alone • -~- ,in flock o fZ22I CFP o o o o 0 0 o

• • e. o o 0 o 0 0 o 0 o 00 o Fig. 1. Distribution of flock crows during spring in relation to a CFP o o and to nests. Sites of observation of a female that nested close to the o CFP are also given. 1km 64 SOCIAL ORGANIZATION HOODED CROW [Ardea73 was usually separate from others and bred, it tions may not be quite comparable as the terri­ was classified as a pair of territorial crows (Fig. torial female spends much of the time on.the 1). nest or in its immediate vicinity and not with the During spring and summer, flock crows usual­ male. I thus think that some of the 15 single ly roosted within one or two kilometres from prospecters really were unmated birds. their daytime feeding areas. Observations of Records of two radio-tracked male crows fur­ two radio-tracked flock crows suggest that ther illustrate these points. One single prospect­ changes of roosting place were common (Figs. ing male was tracked for three days in March 2A, 3A). Most of the day was spent feeding at 1975. He spent about half of each day alone one or two CFP but some flock crows did range within a restricted area, the other half in a flock further afield (Fig. 2B). Such excursions were at one or two places about one km away (Fig. made by single birds or small flocks. 3A). From 1976 onwards he occupied a territory close to the area where he had been prospectc 4.2. ACQUISITION OF TERRITORIES AND PAIR ing. In April 1975 the area where the single BOND male had been prospecting was occupied by an­ other radio-tracked crow that, with its mate, 4.2.1. Territory acquisition by flock spent the whole day there and was thus territori­ crows al. This pair did not breed that year. It roosted Flock crows were sometimes observed alone or in pairs. During winter these observations were more or less randomly scattered among all places where flock crows were observed. How­ ® ever, during spring single or paired flock crows were often observed outside the area usually frequented by flocks. Flock crows that were ob­ served in such situations will be called "prospec­ tors". This pattern is reflected by the relative distance of crows in flocks and prospectors to 5h the nearest CFP during winter January­ March) and spring (April-June). Considering all observations of tagged individuals, these dis­ tances were 770 m (n = 180) and 1100 m (n = 10) during winter (not significant, Median test,x2 = 0.11) and 520 m (n = 155) and 1300 m (n = 25) during spring (significant, Median test, X2 = 5.75, P < 0.05). The flock area fre­ * ® quented by prospectors was the one closest to the area of prospecting (13 cases). Further­ more, if a crow had been observed in a flock area the year before it became territorial, this had been in the area that was closest to its newly acquired territory (5 cases). Prospecting was of­ 1km ten followed by the occupancy of a territory at the site of prospecting, or close to it, in the fol­ lowing year (4 cases; territories were 100, 200, 200 and 400 m from the site of prospecting). Fig. 3. A. Activity range of a prospecting male, radio­ Prospecting crows were single birds in 15 out of tracked on 24 March 1975. It had asimilar range on 23 and 24 cases (62%). This was similar to the fraction 27 March. B. Activity range of a mated male, presumably a first-year of territorial crows that were observed singly territorial, radio-tracked on 3 March 1975. It had a similar during spring (Fig. 8). However the two frac- range on 27 April. Symbols as in Fig. 4 1985] SOCIAL ORGANIZATION HOODED CROW 65 with flock crows about two km from the territo­ flock area at this time; Other pairs, probably the ry (Fig. 3B). majority, formed in the territories. All territori­ als that were known to have lost their mate 4.2.2. Age of newly established (eight males and one female) kept their territo­ territorials ries in the following spring. If the loss occurred None of the sixty crows that had been tagged during the breeding season (seven cases), the as nestlings and that were still in the study area widowed crow sometimes remated in the same with identifiable tags, had established a territory season. This was observed in four cases, includ­ when about one year old. At an age of about ing in the female that lost her partner. In two two years, 2 out of 30 and at about three years, cases no remating was observed until the follow­ 5 out of 13 did possess a territory. Of the former ing breeding season, but such rematings may two, one was breeding and of the latter five, have gone unnoticed. For one case no pertinent three were breeding in their first year as territo­ information is available as the territory was dif­ rials. ficult to observe.

4.2.3. The start of breeding 4.3. ATTACHMENTTOTHETERRITORY Of 12 newly established territorial pairs, sev­ en did not breed in their first year of territorial 4.3.1. Territorials life; five of these seven did so in their second Practically all territorial crows were in their year but two did not. Those that had started territories during April to June (Fig. 4). By defi· breeding continued to do so in succeeding years. nition all should be, but there are a few observa­ In eleven instances one bird was recorded tions that I consider exceptional (section 4.1.1). breeding for the first time while its partner had In years with a late start of breeding some terri­ already bred in the territory previously. In nine torial crows were seen in flocks away from their out of these eleven cases did the newly formed territories during the first week of April; July pair consist of two first time breeders. However, and August were transition months with an in­ most of these pairs had been present in the terri­ creasing number of observations of territorials tory for nearly one year while pairs of inex­ from outside the territory, though mostly in its perienced birds sometimes had been present for vicinity (Fig. 4). About 25% of all observed ter­ less than a month only before the start of breed­ ritorials were in their territory during the winter ing. months. There was variation between pairs with respect to the tendency to stay in their territory 4.2.4. Mate fidelity during winter. I found no correlation between Of 16 different pairs, both partners were experience as territorial (first-year territorials tagged and these were observed for a total of 34 pair-years. In these years, only one pair split up. % This pair had established a territory but no 100 ---_...-----. breeding took place. As both had been tagged in that territor, it is uncertain whether they were first-year territorials but this seems likely as 50 they did not breed (section 4.2.3). The two crows of this pair were mated with untagged partners next year. The male was still in its orig­ inal territory, the female in an adjacent one. JASONDJ FMAMJ 73 72 83 115 121 95 120 133 222 433 255 211 4.2.5. Pair formation Fig. 4. Seasonal variation in the distance at which territori­ Some pairs probably formed between two als were observed from their territory. The drawn line gives flock crows. This was most likely the case with the fraction (%) of all observations within the territory, the broken line the fraction of observations within 1 km of the two crows that prospected together at several territory border. The number of observations is given below places in one spring and were also seen in the the x-axis. 66 SOCIAL ORGANIZATION HOODED CROW [Ardea 73 vs. more experienced ones) or body weight of Table 1. Seasonal variaton in the association of juveniles the crow and the proportion of all observations with their parents. N refers to the number of observations of tagged juveniles of which the mother or father was also made within the territory from October to tagged. "% with mother/father" gives the proportion of March. However, there was a tendency for ter­ these observations when a parent was recorded in the vicini­ ritories that were likely to yield an adequate ty of the juvenile food supply during winter to be more adhered to Month N % with N % with than others. Such territories, e.g. those in the vi­ mother father cinity of places where cattle were fed during hne 13 31 22 9 winter, those containing small streams, one July 13 38 19 37 close to a small village and even one including August 16 0 28 21 September 13 15 15 20 the ecological field station, were significantly October 13 46 15 7 more frequented during winter. (Mann-Whitney November 11 9 18 0 V-test, with territories ranked according to the December 78 0 86 3 proportion of all observations of territories made within their territory during winter, V = 235, P < 0.01). with their parents in flocks during autumn but there was almost no association after November 4.3.2. Juveniles (Table 1). Juveniles were virtually confined to their par­ Two juveniles, of different parents, were aty­ ents' territories in June (Fig. 5). However, com­ pical. During their second and third calendar paratively few juveniles were observed together years they were both still associated with their with their parents in this month (Table 1). This parents' territories during the breeding season was because most juveniles left the nest in the (Figs. 5, 6). At least one of these two crows beginning of June and spent their first weeks in seemed less restricted to the parents' territory its vicinity, waiting for their parents to feed when two years than when one year old (Fig. them. Later, in July, they followed their parents 6B), still it was more associated with it than but fed mainly by themselves. During July and were "typical" juvenile crows (Fig. 5). Both August, increasing numbers of juveniles left these crows were heavier at fledging than aver­ their native territories and almost none were ob­ age (480 and 510 g, average 460 g) suggesting served there from September onwards (Fig. 5). they were males. Juveniles were sometimes observed together 4.4. SEASONAL VARIATION IN FLOCK SIZE AND FEEDING STATIONS % 100 -49 00 -.... _""' Flocks were largest in winter and smallest \ \ during the breeding season (Fig. 7). Though ter­ \ \ ritorials and flock crows were often seen in the 50 \ same flocks there was a tendency for territorials "\ '...... ------...... to be in smaller flocks. Juveniles were mainly ...... _------together with their parents in July and August and the size of the flocks in which the two cat­ JJASONDJ FMAMJ 90+2 69+1 95+3 59+1 45+4 53+4 30+3 50+3 44+7 57+3 90+3 56+9 40+6 egories were observed was similar. Later, juve­ (68) (59) (59) (45) (30) (32) (24) (32) (27) (31) (43) (31) (19) niles behaved more like older flock crows with Fig. 5. Seasonal variation in the distance at which juveniles respect to flock size. In late winter, juveniles were observed from their natal territory. The drawn line re­ were even found in larger flocks than older flock fers to observations within the territory, the broken line to observations within one kilometre away from it. The circles crows. The fraction of crows seen in pairs or refer to two juveniles that were regularly observed in their alone during different seasons showed a similar parents' territory during the spring when they were about pattern (Fig. 8). Older flock crows very rarely one year old. Below the x-axis the number of observations is were alone or in pairs during the period October given, excluding the two atypical juveniles plus the number of observations of these latter; the number of individual ju­ to December and thus showed no tendency to veniles involved is given in parentheses. prospect at this time. Juveniles observed alone 1985] SOCIAL ORGANIZATION HOODED CROW 67

in November (Fig. 9D). Rabbits that had been killed by cars were used as a food source in De­ ® cember and January, usually when snow cov­ ered the ground (Fig. 9C). There were differ­ o ences in these respects between the three cat­ egories of crows. Comparing the two most important feeding site categories, CFP and fields without concentrations, flock crows fre­ quented CFP more than did territorial crows (36%, N = 615 and 4%, N = 1761 respective­ • ly). There was no obvious seasonal variation in o this respect; the proportion of territorial crows at CFP varied between 0 and 18% and that of cP flock crows between 19 and 44%. In both cat­ o ® egories, a maximum occurred in the period No­ @ vember-December. Juveniles behaved similar to territorial crows during their first months (13%, N = 139, at CFP in July-August) but more like older flock crows later on (40%, N = 484, during the rest of their first year). • @Nest 00 Alone or with During midwinter and early spring they were parents •• In a flock even more associated to CFP than were older flock crows (56%, N = 191 during January to April). These tendencies should be compared to Fig. 6. Spring (April to June) distribution of two crows (square and circular symbols) that remained associated with another juvenile tendency, viz., to stay with the their natal territories. The situation in their second (A) and parents in July and August (Table 1) and to the third (B) calendar-year is shown. The father's (drawn poly­ difference in size between flocks joined by juve­ gon) and mother's (broken polygon) home-ranges are given. niles and older flock crows during the period Only one parent was tagged in each pair and the tagged fa- ther had lost its tag in the third calendar-year. . January to March (Fig. 7). or in pairs (the partner may well have been a si­ Ind bling or a parent) were usually still in their na­ 30 tive territory during late summer or autumn. Territorial crows were usually alone or in pairs when observed in their territories (73%, N = 1193). This was especially pronounced 20 during the breeding season (88%, N = 846). When far from their territories (> 1000 m) they were nearly always in flocks (only 5% single or 10 in pairs, N = 322). This fraction was similar to that for flock crows (Fig. 8). Most crows fed in places where no obvious concentration of food was found (Fig. 9E). A' J A S 0 N D JFM A MJ T 20 38 51 85 83 62 100 95 161 67 14 20 considerable proportion was seen at CFP during ... J 41 80 50 49 45 30 43 41 40 87 62 41 -F 25 29 37 68 76 45 47 35 87 78 46 21 winter (Fig. 9A). In some months, many crows --- C 605 1017 1037 1594 1858 2774 -2537 2553 2143 1078 202 229 utilized fields where, for a short period of time, food was plentiful as well as well dispersed .over Fig. 7. Mean size of flocks. T: Territorials. J: Juveniles, F: Flock crows, C: All crows observed during weekly censuses, an extensive area, e.g. newly sown fields in regardless of whether tagged or not. Crows observed singly April and field where manure had been spread or in pairs excluded. Number of crows given below x-axis. 68 SOCIAL ORGANIZATION HOODED CROW [Ardea 73

% N 100 280

240 t, 5.3.1975 I " 50 I , 200 I I " I ,..----<0, I I • I I \ C ~ 160 ,/ I I F 20,2.1975/ I 120 A l T 81 67 ------J 83 113 80 "i -F 28 28 ,--J i ---- C 748 1083 30.1.1979/ t \ 40 -~' Fig. 8. Fractions (%) of crows observed alone or with fami­ ...... / \ \ ly (mate, parents, young of the year). Letters as in Fig. 7. . Number of observations given below x-axis. 16" 17" 18" Time

Fig. 10. Number of crows (N) at different days on evening 4.5. DISTRIBUTION OF CROWS DURING WINTER gathering places, from arrival of the first crows to departure for the roosts. 4.5.1. Size, distribution, and characteristics of roosting places 4.5.2. Flights to and from roosts Most crows were in large roosts during winter The crows usually arrived at the roosts about and these roosts remained in the same place 30 min after sunset. The final flight to the roost from year to year. Crows sometimes also roost­ usually started from one or a few gathering ed in smaller groups during winter as exempli­ places in the neighbourhood (Fig. 11). In winter fied by one radio-tracked crow (Fig. 13B). All several hundred crows, and also Jackdaws and three large winter roosts in the study area were Rooks, left the gathering places together. in small plots of spruce forest as were two out of Roosting places were traditional ones, changing three other known roosts in the vicinity of the position a few hundred of metres at most within study area. or between winters. The number of crows at a gathering place usually increased gradually dur­ ing the hour before roosting. Departure for the %

50

0-- _

JASONDJ FMAMJ 748 1083 1124 1778 2037 29502677 2717 2342 1304 499 409

Fig. 9. Fractions (%) of all crows using distinct types of feeding site in the course of a year. A: Places with concen­ trated food of permanent or long-term availability (CFP). B: Places with concentrated food available for more than a * Roost few days. C: Places with concentrated food available for one * Small roost or a few days. D: Extensive but temporary feeding sites with plenty of food. E: Other sites, mainly fields with no appar­ ent concentrations of food. F: Food. Number of crows ob­ Fig. 11. Location of large, permanent winter roosts and of served given below x-axis. some smaller roosts, used irregularly during winter. 1985] SOCIAL ORGANIZATION HOODED CROW 69

Fig. 12. Roost-recruitment areas. Daytime feeding sites are indicated e. •• for all tagged crows identified at • each of four gathering places (large • symbols) during October-March. • • Included are all sites where these birds were observed within one month from the date when first seen at a gathering place. Use of a longer time interval would have increased the possibility of confusion due to occasional changes of gathering place within one season. Roost-re­ cruitment areas are separated by 1km thick lines, based on the distribution of the observation sites. Crows flew I to the same roost from two of the ...... _--""',I gathering places. roost, on the other hand, took only about ten the feeding places in the morning than on the min for the whole flock (Fig. 10). Crows arriv­ evening flights away from these. ing at the gathering places sometimes from tra­ ditional sub-gathering places, usually came in 4.5.3. Roost-recruitment and winter flocks that were larger than those observed dur­ activity range ing daytime feeding. Gathering places, and also Crows utilizing gathering places in winter traditional sub-gathering places, were situated were observed feeding in different areas during 100--300 m from the CFP (Fig. 11). The flight daytime. These areas are here termed roost-ree pattern in the morning was approximately the cruitment areas. These were roughly adjoining reverse of that in the evening. The same gather­ (Fig. 12). The home-range of an individual crow ing places and sub-gathering places were used. usually covered only a part of one roost-recruit­ However, less time was spent before reaching ment area. Some crows changed recruitment

Table 2. Frequency of roost-recruitment area change (see Fig. 12) for different crow categories during different seasons. A: Per­ centage of observations in the area where each individual crow was observed most frequently. Band C: Percentage in the second and third most frequented area for each crow. N: Total number of observations October-March April-June July-September N AB C N ABC N ABC Juveniles 297 95 4 1 198 89 8 3 233 96 4 0 Flock crows 423 95 4 0 213 95 5 0 106 97 3 0 Territorial crows 829 99 1 0 903 100 0 0 230 100 0 0 70 SOCIAL ORGANIZATION HOODED CROW [Ardea 73

Table 3. Frequency of observations for various categories of crows during winter. The expected model distribution is also given in parentheses together with x2-value for a test of fit between observed (zero-truncated) distribution and the model distribution. For juveniles and flock crows, the Poisson distribution served as a model; for territorial crows the distribution obtained by taking the mean value of the Poisson distribution and the Geometric distribution. These models gave the best fit to the data Number of observations during winter Number of individuals observed the given number of times (October-March) Juveniles Flock crows Territorials o 26 ( 1.3) 12 ( 0.5) 11 (14.8) 1-2 11 ( 8.6) 8 ( 5.9) 27 (28.7) 3-4 10 (11.3) 8 (10.7) 32 (28.3) 5--6 3 ( 5.5) 8 ( 7.2) 13 (17.8) ?:.7 3 ( 1.7) 2 ( 3.3) 16 (11.7) X2 2.95 2.03 3.46 d.f. 3 3 3 F 0.5-D.3 0.7-D.5 O. 5---'-D. 3 area in the course of one winter. This was most feeding sites. Mean flock size at a CFP in De­ cornman for juveniles, less cornman for older cember to February was 27, at other sites 6.3 flock crows and least for territbrials (Table 2). birds. Furthermore, most winter horne-ranges Juveniles and older flock crows sometimes comprised at least one CFP. changed roost-recruitment area during spring and summer but territorials never did during 4.6. MIGRATION these seasons. During winter, the crows usually A large fraction of the juveniles and older spent the whole day at one or two CFP, usually flock crows that were tagged before winter and in large flocks (Fig. 13). They sometimes moved observed later one were never observed during more extensively in smaller flocks within the the winter months (October to March) (Table roost-recruitment area (Fig. 13B). These data 3). This suggests they had left the study area. from radio-trackings are corroborated by infor­ Practically all territorials were observed in the mation on the size of flocks at different types of study area during winter and I conclude that most, probably all, stayed. Twelve ringed crows were recovered dead or shot during winter out­ side the study area. Five of these were found in ® (islands of Sjaelland, Lolland and Fyn), the other seven in the Swedish province Sk{me, where the study area is situated. All sev­ en summer recoveries outside the study area carne from Skane. The latter probably represent permanent emigration while the winter recov­ eries may have resulted from both emigration ® and temporary migration. Out of 19 crows trapped and ringed during winter, three were 1/2h 5 later shot in Finland. ~ /----,- ______~1/2h-""" " ...... 4.7. SURVIVAL ------~- .... _--- ,) ..ljl.------Survival was calculated as the proportion of ,\ ~ birds observed in the study area during one breeding season (April to June) and observed again during the next season. Only crows whose Fig. 13. Daily actIvIty ranges of two crows radio-tracked tags were not older than two years were includ­ during winter: A was tracked on 26 and 27 December 1975 ed as tag loss was neglegible during this period and B on 2 and 7 January 1977. Symbols as in Fig. 4. Dura­ tion of stay is not given when entire day was spent in one of time. These values thus represent minimum place. survival as missing crows may have emigrated. 1985] SOCIAL ORGANIZATION HOODED CROW 71

Survival was 92% for territorial crows (based on However, it seems likely that single birds pros­ 57 birds and 132 bird-years); some birds were pect too as was also suggested by Kalchreuter re-tagged in the course of the study. For flock (1971a). If these single birds are successful in crows (exluding the first year of life) survival establishing a territory, they probably find a was 73% (bases on 54 birds and 62 bird-years). mate in this territory. However, successful terri­ Two of the territorials that disappeared and one tory-establishment by single birds was never ob" flock crow are known to have been shot during served and it is difficult to see how this would be winter, while the fate of the others remains un­ possible in competition with prospecting pairs. known. It may be significant that the single male that was radio-tracked while prospecting lost his po­ 5. DISCUSSION tential territory to a pair. It is possible that pros­ 5.1. SEGREGATION OF TERRITORIES AND FLOCK pecting by single birds is beneficial to them by AREA becoming acquainted to the area, something In spring most flocks were found at or around that may be useful when prospecting with a a CFP. This phenomenon was also observed by mate later on. Charles (1972) and Bohmer (1976). Territorial All widowed males (and one such female) in pairs probably have no influence on the distribu­ my study area were able to maintain their terito­ tion of flocks (Bohmer 1976). Flocks were also ries and form new pairs. Pair-formation most found elsewhere, at places away from crows' likely took place within the territory. This is in nests. This agrees with Charles' (1972) observa­ contrast to Charles'(1972) observation that a tion that "group intrusions" mainly take place at single crow cannot defend its territory. If the fe­ the borderline between territories. The fact that male died during the breeding season, flock territorial crows restrict the movements of flock crows appeared in the territory (Goransson & crows· outside the flock area was also evident Loman 1982). This could actuallypenefit the from an experiment; after territorial females single male, enabling him to find a' new mate were removed in May, flock crows invaded in quickly without having to leave its territory the territories (Goransson & Loman 1982). open to competing pairs. Charles (1972) regularly observed "group in­ trusions at nests" too. This was not recorded in 5.3. TERRITORY ESTABLISHMENT my study area, which may explain the low rate Evidently, the occupancy of a territory is a of predation on eggs and chicks (Loman 1980) prerequisite for breeding. Besides, territory as compared to that recorded by Charles (1972) ownership might increase survival. However, and Wittenberg (1968). They suggested that the figures showing higher survival rate for ter­ flock crows were responsible for this predation. ritorial crows than for flock crows are biassed by The fact that pairs nesting in the flock area differences between both groups in the tenden­ raised young to fledging suggests that predation cy to emigrate. Also, the difference could sim­ from flock crows was of minor importance in my ply be due to higher survival rate of individual study area. The reasons for these differences crows that are particularly fit and therefore are are not clear. the one's most likely to establish territories. The function of evening gatherings of territo­ A young non-territorial crow has two options: rial crows remains obscure; the phenomenon to remain in the natal territory most of the time, may constitute some kind of defence against using it as a base for acquisition of its own terri­ predation, similar to the function suggested for tory, or to join a flock and spend most of the pre-roost gatherings (section 5.7). time in the flock area. The advantages and dis­ advantages of the two options are discussed in 5.2. PAIR FORMATION section 5.5. Most previous investigators have stated that Do flock crows select a preferred territory or pairs are formed in flocks (Wittenberg 1968, do they settle more or less at random? The situ­ Kalchreuter 1971a, Charles 1972). This was also ation in my study area suggests that there prob­ true in my study area, at least in some cases. ably is little opportunity for birds to choose at 72 SOCIAL ORGANIZATION HOODED CROW [Ardea 73 all. Competition for a territory was intense as their territories during summer but return later few teritorials died, leaving vacancies to the in autumn. number of flock crows present. The availability If the territorial area that I studied really of­ of territories may have been even more limited, fered relatively little food, especially as com­ since flock crows usually restricted prospecting pared to the flock area, this could explain three to one roost-recruitment area. Within the study other differences between my study and others': area crow territories are situated in different 1. Kalchreuter (1971a) and I found that juve­ habitat types. In dry areas clutch size has been niles left their natal territories in late summer shown to be relatively small (Loman 1977). while Charles (1972) and Tompa (1975) found Since no differences have been found in breed­ that they stayed in the teritory for a longer peri­ ing experience of crows inhabiting different od of time, to leave it only gradually during habitats (dry vs other), clutch size differences their first winter. A difference between my find­ can be regarded to reflect differences in habitat ings and those of Kalchreuter (1971a) is that he quality. It is striking that territorial crows were found that juveniles formed exclusive flocks in not found to attempt exchanging their territory summer. I found no indication of this; juveniles for a better one. This could be because the es­ were found in flocks with older flock crows and tablishment of a new territory implies less atten­ teritorials. 2. In Charles' study, flock crows es­ tion for the one already occupied, with the risk tablished small and temporary territories during of loosing both territories. These observations winter and early spring. Later on, during the stress the difficulties crows have to obtain terri­ breeding season, they were usually back in the tories. Despite the comparatively high survival flocks. I did not observe this. 3. Charles (1972) rate (for ) of flock crows, it is likely also found that persistent intruders were toler­ that many never succeed in acquiring a territo­ ated during winter as third birds in the territo­ ry. ries but they were evicted before the following breeding season. This was not observed in my 5.4. OCCUPANCY OF TERRITORIES DURING study area. WINTER The tendency for territorial crows to stay in 5.5. JUVENILES STAYING IN THEIR PARENTS' the territory during winter differs between the TERRITORIES various populations studied. In Scotland Only Tompa (1975) has previously observed (Charles 1972, Spray 1978), Switzerland (Tom­ crows, apart from the pair, that stayed perma­ pa 1975), and northern West Germany (Witten­ nently in a territory during the breeding season. berg 1968) territorials stay and defend their ter­ Two out of 36 territories observed by him held ritories throughout the winter. In southern West three crows. He had no further observations Germany the crows remain in their territories concerning origin or nature of these birds. The most of the winter but depart under snow condi­ third birds that Charles (1972) observed were tions (Kalchreuter 1971a). In my study area obviously of different status. They were recruit­ most crows were confined to their territories on­ ed from the flock and evicted by the start of the ly in the period April to June. There was little breeding season. I observed juveniles that has snow but it is possible that during winter there some connection to their natal territory were less opportunities for feeding than in the throughout the winter and, like their parents, three study areas mentioned above as these became a permanent resident there during the were situated in farming areas. Lack of food breeding season. The function of this behaviour cannot explain the crows' absence from their for juveniles and parents is not clear but I will territories during summer in my study, but it is discuss some possibilities. 1. The juvenile that possible that new territories are seldom estab­ remains in the parents' territory could serve as a lished during this season and thus there is no helper (Skutch 1935), defending its parents' ter­ need to defend a territory. This is supported by ritory and assisting with feeding the new brood. Wittenberg's (1968) finding that territorial No observations were made specifically to de­ crows (in his study population) are absent from termine this, but at least in one of the territories 1985] SOCIAL ORGANIZATION HOODED CROW 73 under observation, the juvenile was never ob­ during night (Gyllin et al. 1977). 2. Communal served in the vicinity of the nest, making it un­ roosting could serve as a defence against preda­ likely that it was feeding its younger siblings. 2. tors. This could be accomplished in two ways: a) The juvenile may be better off with respect to Detection of predators is probably facilitated food in the parents' territory than having to where a large number of crows has gathered. b) compete with other crows in the flock area. By If the predators are more or less territorial, staying within the territory, the juvenile crow communal roosting may result in a reduction of may even be able to occupy part of it as a new the number of predators present relative to the territory, especially if the parents' territory is number of crows (Bertram 1978). Gathering large. This was observed for Scrub Jays ApheIo­ places are always situated in open landscapes coma coeruiescens by Woolfenden & Fitzpatrick suggesting defence against predators to be an (1978). By having a stable base in the territorial important factor. Roosts, on the other hand,are area, establishment of a separate territory could situated in copses where potential predators like be facilitated. Charles (1972) reports that dur­ the Goshawk Accipiter gentilis and the Great ing winter, juveniles remaining in the natal ter­ Horned Owl Bubo bubo will have good oppor­ ritory were better tolerated by neighbouring tunities to strike unnoticed. Gathering places territorials than were unfamiliar crows. If this could be important because they allow crows to applies to my study area in spring, prospecting gather at a comparatively safe place and reach by these juveniles would be facilitated. This hy­ the roosting place simultaneously. If they ar­ pothesis is supported by the fact that one of the rived singly at the roost, those arriving first juveniles observed to remain in the natal territo­ would face the risk of being taken by predators. ry established a territory of its own about 500 m Interrupting their flight to the roost at a safe from its parents' territory when three years old. place, and only continuing when sufficiently many other crows are likely to follow, may have 5.6. DISTRIBUTION OF FEEDING SITES DURING started the evolution of a gathering place sys­ WINTER tem. I think, the pattern of arrival to and depar­ In general, juveniles feed mainly at CFP and ture from the gathering places supports this in large flocks during winter while territorials (Fig. 10). 3. Individual crows which have failed visit CFP less frequently and feed in smaller in finding food at a particular day, may follow flocks. Older flock crows appear to be interme­ other crows when departing from the roost next diate in these respects. Flock size is probably morning. In this way roosts may function as an strongly influenced by the nature of the feeding information centre for food-finding (Ward & site. Territorials and older flock crows, which Zahavi 1973). It has been previously suggested are dominant and therefore are able to compete that this is not likely to play an important role in successfully for food, could be expected to be my study area, except perhaps under very se­ more often present at a CFP. However, the re­ vere weather conditions (Loman & Tamm verse is true. I suggest this is because territorials 1980). need to guard their territory, whereas older There was no congruence between roost-re­ flock crows which are capable to defend a va­ cruitment areas and winter home-ranges, thus cant territory should be informed about the situ­ they cannot be considered group territories. ation in the territorial area. This is hardly surprising as both areas are likely to be determined by different factors. Home­ 5.7. FUNCTION OF ROOSTING BEHAVIOUR ranges can be expected to be primarily deter­ Several positive effects on survival of roosting mined by food-availability, whereas roost-re­ behaviour can be envisaged: 1. A roost utilized cruitment areas seem to depend on the distribu­ during winter may be a place with a particularly tion of suitable roosting places, which in turn favourable microclimate, thereby reducing en­ depends on the distribution of predators. Most ergy loss during night (Swingland 1977). This territorial crows in my study area had their win­ cannot be the only function as more energy is ter home-range restricted to part of a roost-re­ lost while flying to the roost than can be saved cruitment area, whereas part of the flock crows, 74 SOCIAL ORGANIZATION HOODED CROW [Ardea 73 especially the juveniles, visited several roost re­ Bohmer, A. 1976. Zur struktur der schweizerischen Ra­ cruitment areas in one winter. The latter proba­ benkrahenpopulation Corvus corone corone. Om. Beob. 73: 109-136. bly were low-ranking birds, which were forced Brown, J. L. 1974. Alternate rol,ltes to sociality in Jays ­ to move from one place to another under the with a theory for the evolution of altruism and pressure of food competiton. communal breeding. Am. Zool. 14: 63-80. Charles, J. 1972. Territorial behaviour and the limitation of population size in the Crow, Corvus corone and 6. ACKNOWLEDGEMENTS Corvus cornix. Ph. D. thesis Aberdeen University, Sam .Erlinge, Hans Kristiansson and Olof Liberg sug­ Goransson, G. & J. Loman. 1982. Does shooting of breed­ gested Improvements to the manuscript. Assistance in the ing Crows influence Pheasant production? - An field was given by many people, especially Thomas Madsen experiment -. Trans. Intern. Congr. Crame BioI. and Niklas Tornlund. The illustrations were prepared by 14: 331-334. Steffi Douwes. Jonathan Thornton and R. van Halewijn Gyllin, R., H. Kallander & M. Sylven. 1977. The microcli­ helped me with the English language. I thank them for their mate explanation of town centre roosts of Jack­ assistance. Grants were received from the Swedish Natural daws Corvus monedula. Ibis 119: 358-361. Science Research Council (to Sam Erlinge) . Kalchreuter, H. 1971a. Untersuchungen an Populationen der Rabenkrahe (Corvus c. corone). Jb. Ges. Na­ 7. SUMMARY turkde. Wiirtemberg 126: 284-339. A population of wing-tagged Hooded Crows Corvus cor­ Kalchreuter, H. 1971b. Untersuchungen an der Krahenmas­ nix was studied in southern Sweden during the years 1974­ senfalle. Z. Jagdwiss. 17: 13-19. 79. Interest was focused on aspects of their social organiza­ Loman, J. 1977. Factors affecting clutch and brood size in tion .and behavioural ecology. Only part of the population the Crow, Corvus cornix. Oikos 29: 294-301. studIed were breeding birds. Breeding pairs were territorial Loman, J. 1980. Reproduction in a population of the during spring when non-breeding crows lived in flocks Hooded Crow Corvus cornix. Holarc. Ecol. 3: mainly in rather restricted areas. If a territorial crow died' 26--35. its mate kept the territory and obtained a new partner: Loman, J. & S. Tamm. 1980. Do roosts serve as "informa­ usually within a month. Pairs which had recently established tion centers" for Crows and Ravens? Am. Nat. a territory had usually spent the previous breeding season in 115: 285-289. the flock-area that was closest to their territory. It was also Picozzi, N. 1971. Wing tags for raptors. Ring 68/69: 169­ common for a pair to visit occasionally an area in the year 170. before they established a territory there. Some crows be­ Skutch, A. F. 1935. Helpers at the nest. Auk 52: 257-273. came territorial at the age of two years but the majority had Spray, C. J. 1978. Territorial behaviour of the Carrion not acquired a territory at the age ofthree. Crow, Corvus corone L., in relation to food supply: Son:e pairs were usually found in their territories through­ An experimental study. Ph. D. thesis. Aberdeen out wmter whereas others were less restricted to it. This dif­ University. ference was probably related to differences between territo­ Swingland, I. R. 1977. The social and spatial organization of ries in possibilities to find food in winter. Most juveniles had winter communal roosting in Rooks (Corvus frugi­ ceased altogether to visit their natal territories by Septem­ legus). J. Zool. Lond. 182: 509-528. ber in the year of hatching. Tompa, F. S.1975. A preliminary investigation of the Car­ I observed two juveniles which to some extent remained rion Crow Corvus corone problem in Switzerland. associated with their natal territories throughout their first Orn. Beob. 72: 181-198. winter. Juveniles fed in large flocks in places with concen­ Ward, P. & A. Zahavi. 1973. The importance of certain as­ trations of food during winter. Territorial crows were usual­ semblages of birds as "information-centres" fot ly observed in smaller flocks and fed in areas without ob­ food-finding. Ibis 115: 517-534. vious concentrations of food. Older flock crows behaved in­ Wittenberg, J. 1968. Freilanduntersuchungen zur Brutbiolo­ termediately in these respects. Most crows frequented large gie und Verhalten der Rabenkrahe (Corvus c. co­ ~mter roosts sItuated up to several km from the crows' day­ rone). Zool. Jb. Syst. 95: 16--146. time feedmg ranges. The crows assembled at gathering Woolfenden, G. E. & J. W. Fitzpatrick. 1978. The inheri­ places before roosting. They left these more or less simulta­ tance of territory in groupbreeding birds. Bio Science 28: 104-108. neously for the roost. Smaller roosts, within two km from feeding places, were used in summer. Some flock crows but no territorials migrated during winter. Most migrants appar­ 9. SAMENVATTING ently wintered in Denmark. In the provincie Skane, zUidelijk Zweden, werd van 1974 Annual survival was at least 92% for territorial crows and tot 19790nderzoek gedaan in een gebied van ruim 20 km2 73% for flock crows more than two years old. aan de 10kale Bonte Kraai populatie. Voor lOver vogels konden worden gevangen werden ze voorzien van ringen en 8. REFERENCES vleugelmerken. In enkele gevallen kon gedurende enige da­ gen het bewegingspatroon van individuele vogels geregi­ Abshagen, K. 1963, Uber die Nester der Nebelkrahen Cor­ streerd worden m.b.v. zendertjes. Het veldwerk bestond vus corone cornix. Beitr. Vogelk. 8: 325-338.' vooral uit het documenteren van het verspreidingspatroon Bertram, B. 1978. Living in groups: Predators and prey. In: en het gedrag van gemerkte vogelsgedurende een aantal J. Krebs & N. Davies (eds.). Behavioural Ecology. opeenvolgende jaren, waarbij gebruik werd gemaakt van Blackwell, Oxford. twee vaste auto-routes in en rondom het studiegebied. 1985] SOCIAL ORGANIZATION HOODED CROW 75

Onderscheid wordt gemaakt tussen territoriale- en ber de binding met de ouders en het ouderlijk territorium. groepsvogels: territorialen worden, tenminste in de periode In twee gevallen werd echter geregistreerd dat jonge vogels april-juni, praktisch uitsluitend gezien binnen een gebied geassocieerd bleven met het territorium van hun ouder, in van beperkte omvang, en solitair of met hooguit een andere hun tweede en derde kalenderjaar. vogel. Aile vogels welke (in april-juni) niet aan dit crite­ In de winter worden territoriale kraaien - voor zover ze rium voideden werden beschouwd als groepskraaien; deze de binding met hun territorium tijdelijk hebben opgegeven kunnen worden aangetroffen in een groter areaal dan terri­ - vooral in kleinere groepen aangetroffen, jonge vogels en toriale vogels, maar toch was hun areaal tamelijk beperkt, groepskraaien in grotere groepen. Beide laatstgenoemde althans in voorjaar en zomer, en wei tot plaatsen met gecon­ categorieen vogels werden in de winter naar verhouding ook centreerde voedselbronnen ("CFP" in de tekst; afvalhopen, veel meer op CFP waargenomen dan territoriale kraaien. mestvaalten, aardappel wintervoorraden, open varkens- en 's Winters frequenteren de meeste kraaien in het studie­ koeiestallen). Arealen van territoriale- en groepskraaien gebied enkele grote, traditionele roestplaatsen gelegen in 6verlappen 's zomers soms enigszins, terwijl arealen van in­ stukjes sparrenbos. Vogels arriveren hier massaal een half dividuele groepskraaien onderling dan in hoge mate over­ um na zonsondergang, komend vanaf een of meer voorver­ lappen. Groepskraaien roesten in voorjaar en zomer op kor­ zamelplaatsen. Deze laatste zijn veelal eveneens traditio­ te afstand van hun foerageergebied en deze vogels ge­ neel, en liggen op korte afstand van CFP, en tot op enige bruiken in de loop van een seizoen meestal meerdere kilometers afstand van de grote roestplaatsen. Bij voorver­ roestplaatsen. In voorjaar en zomer werden groepskraaien zamelplaatsen behoren bepaalde foerageerarealen welke soms enigszins buiten het groepsareaal aangetroffen, en wa­ onderling nauwelijks overlappen. Vooral jonge vogels wis­ ren dan solitair of met een partner. Deze vogels, die meestal selen in de loop van de tijd nogal eens van voorverzamel­ ouder dan drie jaar zijn, worden beschouwd als verkenners: plaats en het daarbij behorende foerageergebied. een jaar later broeden zulke vogels veelal in het eerder ver­ De meeste jonge vogels en groepskraaien waren niet in kende gebied, maar niet aile broedvogels hebben een lange het studiegebied aanwezig tussen oktober en maart. Ring­ verkennersperiode achter de rug. vondsten suggereren dat deze vogels elders in zuid-Zweden Paarvorming kan plaatsvinden tussen twee groeps­ en in Denemarken overwinteren, en dat Finse vogels 's win­ kraaien, maar treedt veelvuldiger op binnen een territo­ ters in het studiegebied voorkomen. rium. Bonte Kraaien blijken in hoge mate trouw te zijn aan De cijfers berekend voor overleving liggen op minimaal hun territorium zowel als aan hun partner. Een kwart van 92% voor territoriale kraaien, en op minimaal 75% voor aile gemerkte territoriale kraaien verbleef het gehele jaar groepskraaien. binnen het territorium; er zijn aanwijzingen dat dit vooral In de discussie worden de Zweedse gegevens vergeleken zo is bij vogels met territoria met een relatief ruim, be­ met resultaten van soortgelijk onderzoek in West-Duits­ trouwbaar voedselaanbod. land, Schotland en Zwitserland en worden deze geinterpre­ lange vogels verliezen in de periode september-novem- teerdin een algemener, theoretisch kader. - R. v. H.